This invention is related, in general, to signal conversion and, more specifically, to signal conversion using a demodulator circuit.
Portable communication products require circuits that can perform well in a low power environment. A reduction of power supply voltages allows for fewer battery cells, reducing the size and weight of the portable equipment. However, the lower power constraint adversely affects the performance of standard RF circuitry. Circuits are needed that can achieve the design goals for noise figure, linearity and power consumption for portable communications products.
In a two-way communication link, the receiver in the portable communication products includes low-noise amplifiers and mixers. The receiver down-converts the received signal in frequency from the Radio Frequency (RF) range to the Intermediate Frequency (IF) range. The output signal generated by the mixer has a frequency that is the difference between the frequency of the received RF signal and the frequency of a local oscillator signal, thus converting the received RF signal to an IF signal.
A demodulator receives the IF signal, demodulates it to baseband and filters the baseband signal. Demodulation usually involves passing the signal through a frequency-dependant phase shift circuit, then comparing the output signal with the original signal in a phase detector or multiplier. The nominal phase shift is 90 degrees, so the nominal output signal from the phase detector is a square wave at twice the input frequency and commonly referred to as the harmonic. Deviations in input frequency produce deviations from phase quadrature which show up as changes in the mark-to-space ratio of the output signal which can be filtered to provide the baseband signal.
However, filtering the unwanted harmonic signal can be difficult under conditions where the amplitude of the unwanted signal is greater than the amplitude of the wanted signal. Accordingly, a need exists for a demodulator in a receiver circuit that generates an output signal having the modulation information while cancelling the unwanted harmonic information.